1. Field of the Invention
This invention relates generally to electrodeposition processes used for making CoFe alloy thin film and more particularly relates to a process which utilizes a low toxic bath at relatively low operating temperatures to produce a CoFe thin film having magnetic properties well suited for the fabrication of magnetic heads.
2. Description of the Relevant Art
Electroplating methods, as well as electrochemical treatment and plating apparatus for the electrodeposition of thin film alloy on a substrate, are well known. For example, Castellani et al, in the U.S. Pat. No. 4,103,756, issued July 25, 1978, teaches methods and apparatus for electroplating Permalloy (NiFe) on a substrate.
According to Castellani et al, a thin film of low magnetostriction Permalloy, of approximately 80% nickel and 20% iron, is electroplated onto the substrate in a bath having a ratio of about 1.8:1 to 24:1 g/liter of Ni to Fe ions with a plating current density of 10 ma/cm.sup.2 to 200 ma/cm.sup.2 when plating in sheet form; and an Ni/Fe ratio of 25:1 to 85:1 with a current density of 2 ma/cm.sup.2 to 110 ma/cm.sup.2 when plating through a mask. The plating bath fluid in the Castellani et al system is constantly mixed, replenished, etc. in a temperature controlled environment to provide the appropriate electrolyte to facilitate the electrodeposition of the desired Permalloy thin film.
Electrodeposited Permalloy thin films have been widely used in magnetic storage applications as recording cores because of their superior magnetic properties such as high saturation moment, near zero magnetostriction, and high permeability.
As recording densities increase, recording media with higher coercivity are needed in order to increase output through reducing self demagnetizing loss. As a result, it is necessary to have a recording core with saturation moment high enough to magnetize such high coercivity media.
In an effort to develop a thin film head for use with high density media with saturation moment and other magnetic properties superior to Permalloy, a variety of thin film alloys and fabrication processes have evolved.
An example of electrodeposition to create a thin film on a substrate using cobalt and iron is taught in Mitsumoto et al, U.S. Pat. No. 4,208,254, issued June 17, 1980. The resulting alloy has a 7.5%-55% iron and 92.5%-45% cobalt composition and is obtained using a plating bath containing a fluoride.
The alloy produced by the Mitsumoto et al technique has high magnetostriction but can be produced using a plating bath requiring a relatively low temperature while plating is in progress. Before Mitsumoto et al, high temperatures, in the range of 80.degree. C. to 90.degree. C. were required to plate CoFe alloy when a bath, composed of cobalt chloride, ferrous chloride and calcium chloride, for example, was used.
For Mitsumoto's end use high magnetostriction was desirable. However, this characteristic is not desirable for magnetic heads. Additionally, the Mitsumoto et al fluoride plating bath is a relatively hazardous, toxic fluid. Accordingly, it would be desirable to find a substitute, relatively low toxic bath for use in CoFe plating yielding a near zero magnetostriction thin film.
Other known CoFe deposition techniques involve dry (non-electrolyte) methods such as vacuum evaporation or sputtering techniques. These vacuum techniques require a relatively high operating temperature, usually in excess of 250.degree. C., and yield films with relatively poor magnetic properties when compared to electoplated films.
In view of the foregoing, it has been determined that it would be desirable to fabricate a CoFE thin film having a saturation moment greater than Permalloy, in a low termperature environment, using an electrodeposition process that does not require a highly toxic bath and which yelds thin film with other overall acceptable magnetic properties for use in fabricating a magnetic head. Such properties include, as indicated hereinbefore, a thin film having, in addition to high saturation moment, near zero magnetostriction, good permeability and a stable magnetic domain.